Such a video server memory management method is known in the art, e.g. from the article "System architecture for a large scale video on demand service" by W. Sincoskie, Computer Networks and ISDN Systems 22, 1991, pp. 155-162, and more particularly from the "NTSC case" discussed in the 2nd paragraph of section 4.1 thereof.
Attention is drawn to the fact that in this article the blocks do not correspond to the above defined blocks as they do not correspond to successive parts of the video signal while on the other hand the above defined blocks are referred to as frames. Also, in the article each of the random access memories corresponds to the memory associated to one of the "parallel heads" as this memory has a single output, i.e. the parallel heads, as mentioned above.
According to this known memory management method a large number of blocks, each corresponding to part of the video signal and together constituting a portion of the video signal, e.g. of 12.5 minutes, are stored in a single random access memory and distinct portions of this video signal are stored in distinct random access memories. Within each of the random access memories use is made of a block interleaving scheme which defines the above mentioned predetermined sequence and permits the optimization of the retrieval rate of a large number of so called predetermined "phases" of a same video signal.
As explained in the article, this management method is optimized for offering a rough video on demand service to user stations coupled to the video server and accepting a limited interactive control, while user stations requesting full interactive control, e.g., VCR functionalities such as fast forward and rewind, are serviced via intermediate stopstart buffers allocated to them.
In other words, full interactive control is with the known method only supported by continuous production of a large number of "phases" of a same video signal and by the use of additional hardware. Although this method is very well suited for some popular video signals, since the continuous production of a large number of "phases" is then justified, for other video signals it is advantageous to allow the user stations to have direct interactive control of the retrieval of video signals from the random access memories, in which case the predetermined sequence corresponds to the sequence in which the video signal is to be displayed.
However, with such direct interactive control the number of user stations allowed to view a same video signal portion stored in a same random access memory, e.g., a hard disk, and retrieved from the same single output thereof is restricted. Indeed, the throughput at this single output is about equal to 20 Mbit/s and the bitrate of a digitally coded video signal typically amounts to 4 Mbit/s which verifiably restricts the number of user stations simultaneously reading information from the same random access memory to 5. In the "NTSC case" discussed in the above article, this restricts the number of user stations simultaneously viewing one of the 8 portions of 12.5 minutes also to 5 while at most 40 user stations are allowed for the complete video signal.
By proceeding in this way each user station may experience serious flaws in the interactive video on demand service. Indeed, in the given example a sixth user station has a worst case waiting time of 12.5 minutes if 5 other user stations just started viewing the first portion of the video signal, and no user station can fast forward or rewind to a following or previous portion of this video signal stored in another memory already used by 5 other user stations.